How Positive-Displacement Pipettes Help to Improve Your Results

Commonly used air-displacement pipettes are highly accurate for most aqueous liquid pipetting applications. However, some research situations exist that are not well-suited for the use of air-displacement pipettes. Liquid properties such as viscosity, volatility, surface tension, or temperature can adversely affect the performance of these pipettes by having an impact on the volume of the air cushion inside the pipette. Additionally, air-displacement pipettes run a higher risk cross-contamination with biological samples and carryover of hazardous or radioactive liquids. For these situations, positive-displacement pipettes, such as MICROMAN® E, are a better tool for the job. They offer optimum performance whatever the nature of the fluid, and also prevent cross-contamination and aerosols.

In this article, we will give you some information about:

• Air-displacement vs. positive-displacement pipettes

• When to use positive-displacement pipettes

• Examples of applications where positive-displacement is better suited for the task

Air-Displacement vs. Positive-Displacement Pipettes

Mechanical air-displacement pipettes, such as PIPETMAN®, have been used in laboratories for over 50 years. These easy-to-use pipettes are based on a simple concept. When the push button is pressed on the pipette, the piston integrated in the lower part of the pipette moves the liquid under the action of an air cushion (called dead volume) between the pipette and the liquid sample. The volume of the air displaced is equivalent to the volume of liquid aspirated (see Figure 1, Panel A).

With positive-displacement pipettes, the piston is not integrated inside the pipette. It is part of the specialized disposable tips called capillary piston (CP) tips and is in direct contact with the liquid. CP tips eliminate the air cushion between the liquid and piston, so volume accuracy is not affected by the fluid’s properties. The disposable capillary piston also prevents contamination since it makes direct contact with the sample, ensuring the user and the pipette are protected (see Figure 1, Panel B). See product guide page 12

Figure 1. Comparison of the Air-displacement and Positive-displacement Pipettes.

Panel A shows the mechanics behind air-displacement pipettes. Panel B describes positive-displacement pipettes.

When to Use Positive-Displacement Pipettes and for What Applications?

With this basic knowledge of the mechanical differences between air-displacement and positive-displacement pipettes, some experimental situations or specific protocol steps requiring the use of certain kinds of fluids can thus be anticipated.The below table gives examples of application where choosing a positive-displacement pipette would be beneficial and summarizes some of the common challenges that can be encountered using air-displacement pipettes with non-aqueous fluids.

Examples Where Positive-Displacement Pipettes Are
Better Suited For The Task

Main Fields

Fluid types


Challenges when using air-displacement pipettes

Molecular biology

Viscous, cold and contaminating

DNA fragments, buffers, enzyme solution, plasmids

• An air bubble may form with foaming liquids or if tip is removed too soon, reducing the aspirated volume

• Pipette shaft is exposed to contaminated air inside, which can transfer to other samples or user.

• Cold liquids tend to be over-dispensed because the air cushion temperature decreases when the tip is placed in the sample causing the air in the tip to condense. This leads to aspirating more sample.

Point of care diagnostic

Viscous, cold and biohazard

Serum, whole blood, plasma

• Cold liquids tend to be over-dispensed

• Pipette shaft is exposed to contaminated air inside

Chemistry, petrochemistry

Volatile, viscous and hazardous

Alcohol, solvents, glue, oil, organic solvents, glycerol

• The liquid can stick to tip, reducing the volume delivered.

• The liquid can flow more slowly, with film adhering to tip walls, decreasing volume dispensed.

• An air bubble may form


Volatile, viscous and contaminant liquids

DNA fragments, body fluids, solvents

• Pipette shaft is exposed to contaminated air inside

• Cold liquids tend to be over-dispensed

Cosmetic and pharmaceutical formulation

Viscous and volatile

Creams, shampoo, drugs, gel

• The liquid can stick to tip, reducing the volume delivered.

• The liquid can flow more slowly, decreasing volume dispensed

Environment and food quality control

Corrosive and volatile

Pesticides, waste waters

• An air bubble may form

• Corrosive samples can evaporate damaging the shaft, seal, and/or piston.

Viscous, hot

Syrup, milk, juices

• The liquid can stick to tip, reducing the volume delivered.

• The liquid can flow more slowly, decreasing volume dispensed.

• Quick evaporation increases internal pressure in tips, leading to leaks.

• Hot liquids tend to be under-dispensed because the air temperature in the tip increases and expands, limiting the volume aspirated.


With air-displacement pipettes, the air cushion between the piston and liquid sample interacts according to the characteristics of the liquid, leading to problems with accuracy (systemic error) and precision (random error).

Positive-displacement pipettes remove that interaction through the use of the capillary piston tips, eliminating the air cushion between the sample and piston so volume accuracy and precision are not affected by the fluid’s properties. Figures 2 and 3 detail data on the improvement in systemic and random error rates with the use of positive-displacement pipettes versus air-displacement pipettes when dispensing a viscous fluid like glycerol.


Figure 2. MICROMAN® E, Positive-Displacement Pipette, Systematic Error.

Figure 3. MICROMAN® E, Positive-Displacement Pipette, Random Error.

MICROMAN E Positive-Displacement Pipettes

Interested in integrating positive-displacement pipettes in your lab?

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